Spin distribution in low-spin (meso-tetraalkylporphyrinato)iron(III) complexes with (dxz,dyz)4(dxy)1 configuration.: Studies by 1H NMR, 13C NMR, and EPR spectroscopies

H-1 NMR, C-13 NMR, and EPR studies of a series of low-spin (meso-tetraalkylporphyrinato)iron(III) complexes, [Fe(TRP)(L)(2)]X where R = Pr-n,Pr-c, and Pr-i and L represents axial ligands such as imidazoles, pyridines, and cyanide, have revealed that the ground-state electron configuration of [Fe((TPPrP)-P-n)(L)(2)]X and [Fe((TPrP)-Pr-c)(L)(2)]X is presented either as the common (d(xy))(2)(d(xz),d(yz))(3) or as the less common (d(xz),d(yz))(4)(d(xy))(1) depending on the axial ligands. The ground-state electron configuration of the isopropyl complexes [Fe(Tt-PrP)(L) (2)]X is, however, presented as (d(xz),d(yz))(4)(d(xy))(1) regardless of the kind of axial ligands. In every case, the contribution of the (d(xz),d(yz))(4)(dxy)(1) state to the electronic ground state increases in the following order: HIm < 4-Me2NPy < 2-MeIm < CN- < 3-MePy < Py < 4-CNPy. Combined analysis of the C-13 and (HNMR)-H-1 isotropic shifts together with the EPR S values have yielded the spin densities at the porphyrin carbon and nitrogen atoms. Estimated spin densities in [Fe((TPrP)-Pr-i)(4-CNPy)(2)](+), which has the purest (d(xz),d(yz))(4)(d(xy))(1) ground state among the complexes examined in this study, are as follows: meso-carbon, +0.045; alpha-pyrrole carbon, +0.0088; beta-pyrrole carbon, -0.00026; and pyrrole nitrogen, +0.057. Thus, the relatively large spin densities are on the pyrrole nitrogen and meso-carbon atoms. The result is in sharp contrast to the spin distribution in the (d(xy))(2)(d(xz,)d(yz))(3) type complexes; the largest spin density is at the beta-pyrrole carbon atoms in bis(1-methylimidazole)(meso-tetraphenylporphyrinato)iron(III), [Fe(TPP)(1-MeIm)(2)](+), as determined by Goff. The large downfield shift of the meso-carbon signal, delta +917.5 ppm at -50 degrees C in [Fe((TPrP)-Pr-i)(4-CNPy)(2)](+), is ascribed to the large spin densities at these carbon atoms. In contrast, the large upfield shift of the alpha-pyrrole carbon signal, delta -293.5 ppm at the same temperature, is caused by the spin polarization from the adjacent meso-carbon and pyrrole nitrogen atoms.